macb: bindings doc: add sifive fu540-c000 binding

Add the compatibility string documentation for SiFive FU540-C0000
interface.
On the FU540, this driver also needs to read and write registers in a
management IP block that monitors or drives boundary signals for the
GEMGXL IP block that are not directly mapped to GEMGXL registers.
Therefore, add additional range to "reg" property for SiFive GEMGXL
management IP registers.

Signed-off-by: Yash Shah <yash.shah@sifive.com>
Reviewed-by: Paul Walmsley <paul.walmsley@sifive.com>
Signed-off-by: David S. Miller <davem@davemloft.net>